The present invention relates to a reflected light barrier having a light source, a transmitting lens, a detector and receiving lens on the same side of a retroreflector wherein the transmitting and receiving lens approximately border on one another, as for example disclosed in German Utility Model Patent No. 73/00004.
A drawback in this known reflected light barrier is that the retroreflector reflects only a small portion of power of transmitted light back onto the receiving lens and then to a detector, whereas a major portion of the transmitted light is reflected onto the transmitting lens placed in front of the light transmitter.
When using retroreflectors composed of many small triple prisms in the form of a grid, it is possible to broaden the transmitted light thereby achieving only a small (few millimeters) parallel offset between the transmitted light beams and reflected light.
Since all real retroreflectors have angle errors in their triple prisms as well as other structural flaws, they all cause slight angular deviations between the transmitted light beams and the reflected light beams and, consequently, a certain amount of radiation travels to the detector via the receiving lens. Therefore, a standard approach may be sufficient for applications requiring only minimal return power. Stated more precisely, the light detector can only detect radiation intercepted by the receiving lens, that is, contained in a descending edge of the received radiation.
FIG. 1 shows a double lens reflected light barrier 1 of conventional design, having a light transmitter or light source 10, a transmitting lens 11 disposed in front of light transmitter 10, a retroreflector 12 in the form of a plastic triple mirror, a receiving lens 13 and a light receiver or detector 14 disposed behind receiving lens 13. Transmitted light beams are marked 15 and received light beams are marked 16.
FIG. 2 is a basic plot showing radiation intensity I(x) of received radiation 16 at values x along transmitting lens 11. Only radiation contained in descending edge 17 of received light beam 16 can be collected by receiving lens 13 and detected by detector 14.
Consequently, so-called "two-eyed" or double lens reflection light barriers ar not particularly suitable either for short or for long ranges. Even for medium ranges, only a small fraction of the transmitted power reaches the light receiver after being reflected by the retroreflector.